Conventionally, axial fans have been used as an air blower for air-conditioner outdoor units.
FIGS. 11 and 12 are a schematic cross-sectional view and a schematic front view of a common outdoor unit for air conditioners, respectively. As shown in these figures, the air-conditioner outdoor unit contains an axial fan A equipped with a plurality of (e.g., three) blades 2, 2, 2 around an outer circumference of a hub 1. On the suction side of the-axial fan A, a heat exchanger B having an L-shaped cross section is placed, while on the discharge side of the axial fan A there is placed a crosspiece type discharge grille C. Reference character D denotes a compressor, and E denotes a partition plate which separates a heat exchange chamber F in which the axial fan A and the heat exchanger B are placed, from a machine chamber G in which the compressor D is placed.
Meanwhile, in one of conventionally well-known axial fans, blades 2 (denoted by the same reference numeral as that used in FIGS. 11, 12 for convenience) have a generally uniform blade thickness from its leading edge 2a to its trailing edge 2b, as shown in FIG. 13 (see, for example, Japanese Patent Laid-Open Publication No. 55-112898).
In such an axial fan, the blade configuration is designed such that air flows to the leading edge 2a of the blade 2 at an optimum angle (i.e., the angle shown by solid-line arrow).
However, in the case of an air-conditioner outdoor unit with the above-described arrangement, since the outdoor unit is closed on the machine chamber G side, air is taken in from two sides of the heat exchanger B so that the direction of air flowing into the axial fan A tends to vary. Also, since frost is formed on the heat exchanger B that is functioning as an evaporator during the heating operation, the direction of air flowing into the axial fan A varies due to a nonuniform resistance to flow caused by the frost formation.
As a result, the inflow angle of air into the blade 2 also varies so that the flow around the blade 2 does not necessarily become an optimum state. That is, when an axial fan having the blade configuration shown in FIG. 13 is adopted as the axial fan A for the outdoor unit of FIGS. 11 and 12, the air enters the leading edge 2a at an angle larger than or smaller than the design angle as shown by broken-line arrows in FIG. 13. Thus, the air flow tends to be separated from the blade surface, resulting in a deteriorated aerodynamic performance and/or an increased aerodynamic sound level of the fan.